<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>watcher</title>
</head>
<body>
  <div id="app">
    {{ user.fullName }}
  </div>

  <script src="../../dist/vue.js"></script>
  <script>
    const vm = new Vue({
      el: '#app',
      data: {
        user: {
          firstName: '诸葛',
          lastName: '亮',
          fullName: ''
        }
      }
    })

    vm.$watch('user',
      function (newValue, oldValue) {
        this.user.fullName = newValue.firstName + ' ' + newValue.lastName
      }, {
        immediate: true, // 立即执行
        deep: true
      }
    )
  </script>
</body>
</html>